Digital variable capacitance circuit, resonant circuit, amplification circuit, and transmitter

ABSTRACT

The present invention aims to provide a digital variable capacitance circuit, a resonant circuit, an amplification circuit, and a transmitter having a high performance. A digital variable capacitance circuit  50  according to this embodiment is a digital variable capacitance circuit including a plurality of unit capacity cells  51 - 0  to  51 - n connected in parallel between two output terminals OUTP and OUTN, in which the unit capacity cell  51  comprises: a first capacitor Cu 1  having one end connected to one output terminal OUTP; a second capacitor Cu 2  that is connected in series with the first capacitor Cu 1  between the two output terminals; and an NMOS transistor M 1  that is connected in parallel with the second capacitor Cu 2  and is controlled in accordance with a digital control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2015-166736, filed on Aug. 26, 2015, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present invention relates to a digital variable capacitance circuit,a resonant circuit, an amplification circuit, and a transmitter.

Japanese Unexamined Patent Application Publication No. 2007-149925discloses a digital variable capacitance circuit that includes aplurality of capacity cells. A switching MOS transistor is connected inseries to the ground side of each of the capacity cells. A P-channel MOStransistor that gives a high-level potential and an N-channel MOStransistor that gives a low-level potential are connected to the gateterminal of the switching MOS transistor. Since the potential level ofthe gate terminal of the switching MOS transistor becomes a low level,the resistance value of the variable resistance element is increased.

In the digital variable capacitance circuit disclosed in JapaneseUnexamined Patent Application Publication No. 2007-149925, however, whena signal having a large voltage amplitude is supplied to an outputterminal, a problem of a breakdown voltage or a leak current may occur.

The other problems of the related art and the novel characteristics ofthe present invention will be made apparent from the descriptions of thespecification and the accompanying drawings.

SUMMARY

According to one embodiment, a digital variable capacitance circuitincludes a plurality of capacity cells that are connected in parallel,in which each of the capacity cells includes a first capacitor havingone end connected to one output terminal, an impedance element that isconnected in series with the first capacitor between two outputterminals, and a transistor that is connected in parallel with theimpedance element and is controlled in accordance with a digital controlsignal.

According to the embodiment, it is possible to provide a digitalvariable capacitance circuit, a resonant circuit, an amplificationcircuit, and a transmitter having a high performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, advantages and features will be moreapparent from the following description of certain embodiments taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing a configuration of a communication apparatusincluding a digital variable capacitance circuit;

FIG. 2 is a circuit diagram showing a configuration of an HPA circuitused in the communication apparatus;

FIG. 3 is a diagram showing a whole configuration diagram of a digitalvariable capacitance circuit according to a comparative example;

FIG. 4 is a circuit diagram showing a unit capacity cell of the digitalvariable capacitance circuit shown in FIG. 3;

FIG. 5 is a diagram showing a current-voltage waveform when a transistorM1 is in an off state;

FIG. 6 is a circuit diagram of a digital variable capacitance circuitaccording to a first embodiment;

FIG. 7 is a circuit diagram showing a unit capacity cell of the digitalvariable capacitance circuit shown in FIG. 6;

FIG. 8 is a diagram showing simulation results obtained by comparingcapacity performances of digital variable capacitance circuits;

FIG. 9 is a circuit diagram of a digital variable capacitance circuitaccording to a second embodiment;

FIG. 10 is a circuit diagram showing a unit capacity cell of the digitalvariable capacitance circuit shown in FIG. 9;

FIG. 11 is a circuit diagram showing a digital variable capacitancecircuit according to a third embodiment;

FIG. 12 is a circuit diagram showing a unit capacity cell of the digitalvariable capacitance circuit shown in FIG. 11;

FIG. 13 is a circuit diagram showing a unit capacity cell of a digitalvariable capacitance circuit according to a fourth embodiment;

FIG. 14 is a circuit diagram showing an LNA circuit to which the digitalcapacitance circuit according to this embodiment is applied;

FIG. 15 is a circuit diagram showing a VCO circuit to which the digitalcapacitance circuit according to this embodiment is applied; and

FIG. 16 is a diagram showing a cross-sectional configuration of athick-film transistor and a thin-film transistor.

DETAILED DESCRIPTION First Embodiment

For the clarification of the description, the following description andthe drawings may be omitted or simplified as appropriate. Further, eachelement shown in the drawings as functional blocks that perform variousprocessing can be formed of a CPU, a memory, and other circuits inhardware and may be implemented by programs loaded in the memory insoftware. Those skilled in the art will therefore understand that thesefunctional blocks may be implemented in various ways by only hardware,only software, or the combination thereof without any limitation.Throughout the drawings, the same components are denoted by the samereference symbols and overlapping descriptions will be omitted asappropriate.

In recent years, a radio frequency integrated circuit (RFIC) thatcomplies with low power consumption wireless communication standardBluetooth (registered trademark) Low Energy (hereinafter it will bereferred to as a BLE) for wearable products has been widely used.Further, the development of smart meter (power, gas) products has beenfocused on achieving an Internet of Things (IoT) society. In accordancetherewith, the development of Sub-GHz RFIC that complies with IEEE802.15.4 g has been advancing.

It is required that the RFICs that comply with the BLE and the Sub-GHzRFICs that comply with a gas meter, in particular, operate over a longperiod of time by the batteries thereof. Currently, RFIC products thatare assumed to be driven by a lithium ion battery (about 3.0 V) having ahigh drive voltage are commercially available. However, lithium ionbatteries are more expensive than other batteries and are lesseconomical. In order to enhance economy, it is required to employ analkaline battery (about 1.6 V) having a low drive voltage. As the trendwhere RFIC products that comply with the BLE or Sub-GHz are widely usedcontinues in the future, a low voltage will become one of the keyproduct requirements.

FIG. 1 shows a configuration example of a communication apparatus 1according to this embodiment. FIG. 1 is a circuit diagram showing aconfiguration of the communication apparatus 1. The communicationapparatus 1 includes an RFIC 100, a Micro Control Unit (MCU)/MicroProcessing Unit (MPU) 200, an antenna 301, and a battery 400. The RFIC100 is a Sub-GHz RFIC. Further, a digital variable capacitance circuit(hereinafter it will be also referred to as a variable capacitancecircuit) according to this embodiment is used as the RFIC 100, as willbe described later.

The MCU/MPU 200 generates data to be transmitted by the RFIC 100.Further, the MCU/MPU 200 executes processing in accordance with datademodulated by the RFIC. The MCU/MPU 200 is connected to the RFIC 100via a predetermined interface.

The RFIC 100 is a Sub-GHz wireless communication apparatus. The RFIC 100includes a controller 11, a MAC MODEM 12, a receiver 20, and atransmitter 40. The RFIC 100 further includes RF input terminals 21 and22 and an RF output terminal 45.

The communication apparatus 1 includes the battery 400. That is, thecommunication apparatus 1 is driven by the battery 400. Therefore, thebattery 400 supplies power to the RFIC 100, the MCU/MPU 200 and thelike. The battery 400 is, for example, an alkaline battery.

The receiver 20 includes circuits of a radio receiving system (RX).Specifically, the receiver 20 includes an internal matching circuit 23,an LNA circuit 24, a mixer 25, an IFA circuit 26, a low-pass filter(LPF) 27, an Analog to Digital (A/D) converter 28, a Voltage ControlOscillator (VCO) circuit 42, and a Phased Locked Loop (PLL) circuit 41.

The transmitter 40 includes circuits of a radio transmitting system(TX). Specifically, the transmitter 40 includes the PLL circuit 41, theVCO circuit 42, an HPA circuit 43, and a band reject filter 44. The PLLcircuit 41 and the VCO circuit 42 are shared by the receiver 20 and thetransmitter 40.

The antenna 301 transmits a high-frequency signal to a space as radiowaves and receives radio waves from the space as a high-frequency signalin order to perform radio communications. The RFIC 100 performsprocessing of the reception signal received by the antenna 301 and thetransmission signal transmitted from the antenna 301. This processingwill be described later.

The RFIC 100 is connected to the antenna 301 via a transmission path TXpath and a reception path RX path. Specifically, a capacitor 303 isprovided in the reception path RX path. The reception signal received bythe antenna 301 is input to the RF input terminal 21 via the capacitor303. Further, the RF input terminal 22 is connected to the ground.

A capacitor 304 and a matching circuit 305 are provided in thetransmission path TX path. The transmission signal generated by the RFIC100 is supplied to the antenna 301 via the matching circuit 305 and thecapacitor 304. Further, an RF switch 302 is provided between the antenna301 and the RFIC 100. The RF switch 302 switches the transmission path(TX Path) and the reception path (RX Path) of the high-frequency signal.

A description will now be given of the transmission processing. Aninstruction to transmit data is output to the RFIC 100 from the MCU/MPU200. Then the controller 11 executes signal processing to RFIC-controlthe instruction from the MCU/MPU 200. The MAC MODEM 12 then generatestransmission signal data. The PLL circuit 41 and the VCO circuit 42modulate the transmission signal data and a carrier wave. For example,the transmission signal data is ΔΣ modulated and the modulated data istransmitted on the carrier wave. The VCO circuit 42 then outputs thetransmission signal that has been modulated to the HPA circuit 43.

The HPA circuit 43 amplifies the transmission signal modulated by thePLL circuit 41 and the VCO circuit 42 and outputs the amplified signalto the band reject filter 44. The transmission power is thereforeamplified. The band reject filter 44 removes unnecessary band componentsof the transmission signal. The transmission signal that has passed theband reject filter 44 is input to the RF output terminal 45.

The transmission signal from the RF output terminal 45 is supplied tothe RF switch 302 via the matching circuit 305 and the capacitor 304.The matching circuit 305 matches the impedance to propagate a high-powertransmission signal output from the HPA circuit 43 to the antenna 301without loss. When transmission is performed, the RF switch 302 connectsthe antenna 301 and the RF input terminal 21 via the transmission pathTX Path. The transmission signal from the RFIC 100 is therefore suppliedto the antenna 301. The antenna 301 emits an electromagnetic wave inaccordance with the transmission signal. It is thus possible to transmitdata by radio.

Next, a description will be given of the reception processing. When theantenna 301 receives the radio waves, the reception signal in accordancewith the radio waves that the antenna 301 has received is propagatedthrough the reception path RX path. That is, the reception signal isinput to the RF input terminal 21 via the capacitor 303. The internalmatching circuit 23 is arranged just before the LNA circuit 24. Theinternal matching circuit 23 performs impedance matching. It istherefore possible to supply power from the antenna 301 without loss.The Low Noise Amplifier (LNA) circuit 24 is a first-stage amplifier ofthe receiving system and amplifies the reception signal received by theantenna 301.

The mixer 25 demodulates the reception signal received by the antenna.That is, the mixer 25 extracts data that has been transmitted on thecarrier wave from the high-frequency signal. The mixer 25 then outputsthe current in accordance with the data that has been extracted to theIFA circuit 26. The mixer 25 performs demodulation processing using alocal signal output from the VCO circuit 42. The local signal is asignal having a frequency the same as that of the carrier wave of thehigh-frequency signal.

An Intermediate Frequency Amplifier (IFA) circuit is, for example, atransimpedance amplifier. The IFA circuit 26 performs a current-voltageconversion of the output current from the mixer 25. That is, since theoutput of the mixer 25 is a current and the input of the LPF 27 is avoltage, the IFA circuit 26 performs the current-voltage conversion.

The LPF 27 suppresses components other than a desired wave included inthe reception signal from the antenna 301. That is, the reception signalalso includes, in addition to the desired wave, a disturbing signal thatis unnecessary for the reception. The LPF 27 allows only the low-bandfrequency to pass therethrough, whereby the disturbing signal issuppressed. The A/D converter 28 AD converts the reception signal thathas passed the LPF 27. Since the MAC MODEM 12 is a digital signalprocessing circuit, the A/D converter 28 generates a digital receptionsignal and outputs the generated signal to the MAC MODEM 12.

Enhancements in the efficiency and the power output of the HPA circuit43 can be achieved when a balun and the capacitor are resonating inparallel. The capacitor needs to have a variable function in order tocomply with the communication standard of each country. FIG. 2 shows acircuit configuration example of the HPA circuit 43 and the band rejectfilter 44.

The HPA circuit 43 includes an amplifier 47, a balun 48, and a capacitorC1. In the balun (balanced-unbalanced converter) 48, an inductor L1 andan inductor L2 are coupled to each other. The band reject filter 44includes a capacitor C2 and an inductor L3. The inductor L2 and thecapacitor C1 are parallel resonant circuits. Further, the capacitor C2and the inductor L3 are serial resonant circuits. The inductors L1 andL2 and the capacitor C1 are balun resonant circuits. The matchingcircuit 305 includes an external capacitor C_(EXT1) and an externalinductor L_(EXT1).

The transmission signal output from the VCO circuit 42 is input to theamplifier 47. The amplifier 47 amplifies the transmission signal andoutputs the amplified signal to the balun 48. The balun 48 converts adifferential signal from the amplifier 47 into a single end signal. Inthe balun 48, the inductor L2 and the inductor L1 are coupled to eachother. The inductor L2 and the capacitor C1 are parallel resonantcircuits. Therefore, the transmission signal input to the balun 48 isoutput at low loss.

The capacitor C1 and the inductor L2 are connected in parallel betweenan RF output terminal 45 a and an RF output terminal 45 b. That is, oneend of the capacitor C1 is connected to the RF output terminal 45 a andthe other end thereof is connected to the RF output terminal 45 b. Oneend of the inductor L2 is connected to the RF output terminal 45 a andthe other end thereof is connected to the RF output terminal 45 b.

Further, the band reject filter 44 is connected in parallel with thecapacitor C1 between the RF output terminal 45 a and the RF outputterminal 45 b. The inductor L3 and the capacitor C2 are connected inseries between the RF output terminal 45 a and the RF output terminal 45b. Specifically, one end of the inductor L3 is connected to the RFoutput terminal 45 a and the other end thereof is connected to one endof the capacitor C2. The other end of the capacitor C2 is connected tothe RF output terminal 45 b.

Further, in the outside of the RFIC 100, the matching circuit 305 isarranged between the RF output terminal 45 a and the RF output terminal45 b. The RF output terminal 45 a is connected to the RF switch 302 viathe external inductor L_(EXT1). The RF output terminal 45 b is connectedto the ground. One end of the external inductor L_(EXT1) is connected tothe RF output terminal 45 a and the other end thereof is connected toone end of the external capacitor C_(EXT1). The other end of theexternal capacitor C_(EXT1) is connected to the ground and the RF outputterminal 45 b.

By resonating the inductor L2 and the capacitor C1 in parallel, theattenuation rate in the balun 48 becomes minimum in a resonancefrequency. It is therefore possible to improve efficiency and poweroutput. Since the frequency band of the resonance frequency definedvaries among countries, it is required to provide a function ofadjusting the frequency band of the resonance frequency. The resonancefrequency is adjusted by making the capacitor variable. That is, byemploying a variable capacitor as the capacitor C1, the communicationapparatus 1 can be used in various countries. It is therefore possibleto employ the variable capacitance circuit according to this embodimentas the capacitor C1.

On the other hand, when the transmission power is high, the voltageamplitude on the RFIC end (e.g., RF output terminal 45 a) increases,which may cause a malfunction of the variable capacitance circuit. In alow power supply voltage operation, in particular, the problem of theabove malfunction becomes more serious. In the following description, anexample in which the RFIC 100 shown in FIG. 1 is used in Japan will bediscussed.

According to ARIB STD-T108, the maximum transmission power on the end ofthe antenna 301 is defined to be 13 dBm or smaller. When there is a lossof 2.5 dB in the RF switch 302, the output power needs to be equal to orsmaller than 15.5 dBm in the RF output terminal 45 of the RFIC 100.Since it is desired to increase the distance from a meter to aconcentrator in a smart meter network, it is required to increase thetransmission output as much as possible. Therefore, it is required thatthe maximum transmission output in the RFIC 100 be 15.5 dBm. When thetransmission power is denoted by Pout and the impedance on the end ofRFOUT in FIG. 2 is denoted by Z_(RFOUT), the voltage amplitude V_(RFOUT)on the end of RFOUT is expressed by the following Expression (1).

V _(RFOUT)=√{square root over (10^(P) ^(out)^([dBm]/10)×1mW×Z_(RFOUT))}  (1)

Consider a case, for example, in which the output power is 15.5 dBm andthe impedance on the end of RFOUT is 100 ohms. At this time, the voltageamplitude on the end of RFOUT is 1.88 V peak voltage from Expression(1). When the transmission output is 15.5 dBm, the voltage amplitude onthe end of RFIC becomes extremely large. An increase in the voltageamplitude may lead to a malfunction of the variable capacitance circuitthat forms the capacitor C1 or the capacitor C2. In the low power supplyvoltage operation, the problem of the malfunction of the variablecapacitance circuit becomes more serious. In this embodiment, themalfunction of a variable capacitance circuit 50 due to a large voltageamplitude is prevented and the variable capacitance circuit 50 accordingto this embodiment is further applied to a low power supply voltage.

FIG. 3 shows a whole configuration of the variable capacitance circuit50 according to a comparative example. In the variable capacitancecircuit 50, n unit capacities 51 are connected in parallel. The symbol nis an integer equal to or larger than two. FIG. 3 shows a configurationin which unit capacity cells 51-0 to 51-n are connected in parallelbetween two output terminals OUTP and OUTN. The capacitance value of thevariable capacitance circuit 50 is a combined capacitance value of theplurality of unit capacity cells 51-0 to 51-n connected in parallel.

In FIG. 3, the output terminals of the respective unit capacity cells51-0 to 51-n are denoted by the output terminals OUTP and OUTN. That is,the unit capacity cells 51-0 to 51-n have two common output terminalsOUTP and OUTN. The output terminal OUTP corresponds to the RF outputterminal 45 a shown in FIG. 2 and the output terminal OUTN correspondsto the RF output terminal 45 b. Since each of the unit capacity cells51-0 to 51-n has a single ended output, the output terminal OUTN isconnected to the ground. A digital enable control signal (hereinafter itwill be simply referred to as a control signal) is input to controlinput terminals b0-bn of the respective unit capacity cells 51-0 to51-n.

The plurality of unit capacity cells 51-0 to 51-n are independentlycontrolled by the control signal. Therefore, the whole capacitance valueof the variable capacitance circuit 50 becomes variable. The unitcapacity cells 51-0 to 51-n have the same configuration except for thecontrol signal to be input thereto. FIG. 4 shows a configuration of theunit capacity cell 51. While the configuration of the unit capacity cell51-0 including the control input terminal b0 is shown in FIG. 4, theother unit capacity cells 51-1 to 51-n also have a configuration thesame as that of the unit capacity cell 51-0. Therefore, a description ofthe configuration of the unit capacity cells 51-1 to 51-n will beomitted.

As shown in FIG. 4, the unit capacity cell 51 includes a first capacitorCu1, an NMOS (Metal Oxide Semiconductor) transistor M1, a resistor R1, atransmission gate TG1, and a NOT circuit NT1. The NMOS transistor M1 andthe transmission gate TG1 each serve as a switch.

The first capacitor Cu1 and the NMOS transistor M1 are connected inseries between the output terminal OUTP and the output terminal OUTN.Specifically, one end of the first capacitor Cu1 is connected to theoutput terminal OUTP. The other end of the first capacitor Cu1 isconnected to the drain terminal of the NMOS transistor M1. The sourceterminal of the MMOS switch M1 is connected to the output terminal OUTN.The terminal between the first capacitor Cu1 and the NMOS transistor M1is denoted by a first intermediate terminal N1.

The control input terminal b0 is connected to the gate terminal of theNMOS transistor M1. The control signal to make the capacitance value ofthe variable capacitance circuit 50 variable is connected to the controlinput terminal b0. Therefore, the NMOS transistor M1 serves as a switchthat is turned on or off by the control signal.

Further, a bias voltage input terminal VB is connected to the firstintermediate terminal N1 via the transmission gate TG1 and the resistorR1. The first intermediate terminal N1 is connected to the drainterminal of the NMOS transistor M1. Further, the negative controlterminal of the transmission gate TG1 is connected to the control inputterminal b0. The positive control terminal of the transmission gate TG1is connected to the output of the NOT circuit NT1. The control terminalb0 is connected to the input side of the NOT circuit NT1.

Therefore, the ON/OFF of the transmission gate TG1 is controlled by thecontrol signal. When the transmission gate TG1 is turned on, a biasvoltage is supplied to the first intermediate terminal N1 and the drainterminal of the NMOS transistor M1 via the resistor R1.

When the control signal input to the control input terminal b0 is H, theswitch resistance of the NMOS transistor M1 becomes zero and the stateof the transmission gate TG1 becomes a high-impedance state. The unitcapacity cell 51 is therefore in an on (active) state and thecapacitance value of the unit capacity cell 51 becomes equal to thecapacitance value (hereinafter it will be denoted by Cu) of the firstcapacitor Cu1. When the control signal is L, the switch resistance ofthe NMOS transistor M1 becomes infinite, the transmission gate TG1 isshorted out, and the drain DC voltage of the NMOS transistor M1 becomesthe bias voltage. The unit capacity cell 51 is therefore in an OFFstate.

When a large voltage amplitude is applied to the output terminals OUTPand OUTN in a state in which the unit capacity cell 51 is in the offstate, the NMOS transistor M1 may malfunction and thus a breakdownvoltage specification may not be satisfied. FIG. 5 shows image diagramsof a voltage waveform and a current waveform applied to the drainterminal of the NMOS transistor M1 in the OFF state.

When the state of the unit capacity cell 51 becomes the OFF state, thevoltage amplitude applied to the variable capacitance circuit 50 isapplied to the drain voltage of the NMOS transistor M1 as it is. This isbecause, since the OFF resistance of the NMOS transistor M1 is infinite,the voltage is not divided and is propagated as it is. At this time, thedrain voltage V(D) of the NMOS transistor M1 is shown by the followingExpression (2).

VB+Vrf≧V(D)≧VB−Vrf   (2)

The symbol Vrf denotes a voltage amplitude. Now, the voltage operatingrange of the drain voltage of the NOMS switch M1 will be discussed. Themaximum value of the drain voltage of the NMOS switch M1 is determinedby an element breakdown voltage Vmax of the off transistor. On the otherhand, the minimum value of the drain voltage of the NMOS transistor M1is determined under the condition that the malfunction in which the NMOStransistor M1 becomes ON does not occur. When a threshold voltage of theNMOS switch M1 that is used is denoted by V_(TH), the minimum value ofthe drain voltage of the NMOS transistor M1 becomes −V_(TH). Therefore,the voltage operating range of the drain voltage of the NMOS transistorM1 is expressed by the following Expression (3).

V max≧V(D)≧−V _(th)   (3)

In order to satisfy Expression (3) and to make the voltage amplitudemaximum, the bias voltage VB and the allowable maximum voltage amplitudeVrf need to be set as shown in the following Expressions (4) and (5).

VB=1/2(Vmax−V _(TH))   (4)

Vrf=1/2(Vmax+V _(TH))   (5)

Consider a case, for example, in which the variable capacitance circuit50 shown in FIGS. 3 and 4 is applied as the capacitor C1 shown in FIG. 2and the RFIC 100 outputs the power of 15.5 dBm. It is assumed, at thistime, that the breakdown voltage specification when the NMOS transistorM1 is in the off state is 2.0 V and the threshold voltage V_(TH) is 0.3V. From Expressions (4) and (5), the bias voltage VB becomes 0.85 V andthe allowable maximum voltage amplitude becomes 1.15 V peak voltage.

As described above, when the output power is 15.5 dBm, the voltageamplitude on the end of RFOUT is 1.88 V peak voltage. At this time, fromExpression (2), the maximum voltage is 2.73 V and the minimum voltage is−1.03 V. This therefore causes the element breakdown voltage of the NMOStransistor M1 and the malfunction of the switch when it is in the offstate.

In order to perform a high output transmission in the above variablecapacitance circuit, it is possible to use a high breakdown voltage MOStransistor for the NMOS transistor M1. When the high breakdown voltageMOS transistor is used, the element breakdown voltage of the transistorand the threshold voltage are higher than those when a thin-film MOStransistor is used.

When the breakdown voltage of the high breakdown voltage MOS transistoris 4.1 V and the threshold voltage is 0.5 V, for example, the biasvoltage VB becomes 1.8 V and the allowable maximum voltage amplitude hasa 2.3 V peak voltage. By using the high breakdown voltage MOStransistor, the RFIC 100 is able to transmit the power of 15.5 dBm. Whenthe power supply voltage becomes equal to or smaller than 1.8 V,however, it becomes impossible to generate the bias voltage VB, wherebyit becomes quite difficult to reduce the power supply voltage. That is,it becomes impossible to use a 1.6 V alkaline battery.

Therefore, in the variable capacitance circuit according to thecomparative example, it is difficult to achieve both the hightransmission output of 15.5 dBm and the low power supply voltageoperation. It is required to provide a new circuit configuration thatnormally operates under such conditions. By using the variablecapacitance circuit according to this embodiment, it is possible toprovide a communication apparatus capable of achieving both the hightransmission output and the low power supply voltage operation.

With reference to FIGS. 6 and 7, the variable capacitance circuitaccording to this embodiment will be described. FIG. 6 is a circuitdiagram schematically showing a configuration of the variablecapacitance circuit 50. FIG. 7 is a circuit diagram showing aconfiguration of the unit capacity cell 51. In the followingdescription, an example in which the variable capacitance circuit 50 isthe capacitor C1 shown in FIG. 2 will be described.

As shown in FIG. 6, the variable capacitance circuit 50 includes aplurality of unit capacity cells 51-0 to 51-n. The plurality of unitcapacity cells 51-0 to 51-n are connected in parallel between two outputterminals OUTP and OUTN. The capacitance value of the variablecapacitance circuit 50 is a combined capacitance value of the pluralityof unit capacity cells 51-0 to 51-n that are connected in parallel.

In FIG. 6, the output terminals of each of the unit capacity cells 51-0to 51-n are indicated by the output terminals OUTP and OUTN. That is,the unit capacity cells 51-0 to 51-n have two common output terminalsOUTP and OUTN.

The output terminal OUTP corresponds to the RF output terminal 45 ashown in FIG. 2 and the output terminal OUTN corresponds to the RFoutput terminal 45 b. Therefore, an output voltage V (OUT) of the balun48 is supplied to the output terminal OUTP. The output terminal OUTN isconnected to the ground. A digital enable control signal (hereinafter itwill be simply referred to as a control signal) is input to respectivecontrol input terminals b0 to bn of the respective unit capacity cells51-0 to 51-n.

The plurality of unit capacity cells 51-0 to 51-n are independentlycontrolled by the control signal. Therefore, the whole capacitance valueof the variable capacitance circuit 50 becomes variable. The unitcapacity cells 51-0 to 51-n have the same configuration except for thecontrol signal to be input thereto. FIG. 7 shows the configuration ofthe unit capacity cell 51. While the configuration of the unit capacitycell 51-0 including the control input terminal b0 is shown in FIG. 7,the other unit capacity cells 51-1 to 51-n have a configuration the sameas that of the unit capacity cell 51-0. Therefore, a descriptions of theconfiguration of the unit capacity cells 51-1 to 51-n will be omitted.

As shown in FIG. 7, the unit capacity cell 51 includes a first capacitorCu1, a second capacitor Cu2, and an NMOS transistor M1. The NMOStransistor M1 serves as a switch.

The first capacitor Cu1 and the second capacitor Cu2 are connected inseries between the output terminal OUTP and the output terminal OUTN.Specifically, one end of the first capacitor Cu1 is connected to theoutput terminal OUTP and the other end of the first capacitor Cu1 isconnected to one end of the second capacitor Cu2. The other end of thesecond electrode Cu2 is connected to the output terminal OUTN. Theterminal between the first capacitor Cu1 and the second capacitor Cu2 isreferred to as a first intermediate terminal N1.

The NMOS transistor M1 and the second capacitor Cu2 are connected inparallel between the first intermediate terminal N1 and the outputterminal OUTN. Specifically, the drain terminal of the NMOS transistorM1 is connected to the first intermediate terminal N1. The sourceterminal of the NMOS transistor is connected to the output terminalOUTN. Therefore, the source terminal of the NMOS transistor is connectedto the other end of the second capacitor Cu2.

The gate terminal of the NMOS transistor M1 is connected to the controlinput terminal b0. The control signal is input to the control inputterminal b0. Therefore, the NMOS transistor M1 serves as a switch thatis turned on or off by the control signal.

When the unit capacity cell 51 is in the off (inactive) state, the drainvoltage applied to the NMOS transistor M1 is capacitively divided. It istherefore possible to attenuate the voltage. Now, an active operation ofthe unit capacity cell will be described. When the control signal inputto the control input terminal b0 is H, the switch resistance of the NMOStransistor M1 becomes zero. Accordingly, the unit capacity cell 51 is inthe on (active) state. Therefore, the whole capacitance value of theunit capacity cell 51 becomes equal to a capacitance value Cu of thefirst capacitor Cu1.

When the control signal is L, the NMOS transistor M1 becomes OFF and theswitch resistance of the NMOS transistor M1 becomes infinite. The unitcapacity cell 51 is therefore in the off (inactive) state. Thecapacitance value of the unit capacity cell 51 becomes the combinedcapacitance value of two series capacitances. When the capacitance valueof the first capacitor Cu1 is denoted by Cu and the capacitance value ofthe second capacitor Cu2 is denoted by M·Cu, the capacitance value ofthe unit capacity cell 51 becomes M/(1+M)·Cu. That is, the capacitancevalue of the unit capacity cell 51 becomes a series combined capacitanceof the first capacitor Cu1 and the second capacitor Cu2.

When the voltage applied to the unit capacity cell 51 is denoted byV(OUT), the drain voltage V(D) of the NMOS transistor M1 is expressed bythe following Expression (6).

$\begin{matrix}{{V(D)} = {\frac{1}{1 + M} \cdot {V({OUT})}}} & (6)\end{matrix}$

As described above, the voltage V(D) that has been capacitively dividedis output to the drain terminal. In the comparative example, V (OUT) isdirectly output to the drain terminal. Therefore, in the variablecapacitance circuit 50 according to the first embodiment, the voltageapplied to the NMOS transistor M1 can be suppressed. It is thereforepossible to prevent the malfunction of the switch of the NMOS transistorM1. Further, it is possible to use the element having a low breakdownvoltage as the NMOS transistor M1. It is therefore possible to form theNMOS transistor M1 by the thin-film MOS transistor. It is thereforepossible to reduce the power supply voltage. It is therefore possible todrive the RFIC 100 by the alkaline battery.

Another advantage of the configuration according to the first embodimentwill be described. The Quality factor, which is one capacity performanceof the capacitance circuit, is greater than that in the comparativeexample. FIG. 8 shows simulation results of the Quality factor and thecombined capacitance value of the variable capacitance circuit 50 in thefirst embodiment and in the comparative example. In FIG. 8, thehorizontal axis indicates the combined capacitance value and thevertical axis indicates the Quality factor. In FIG. 8, the simulationresults of the first embodiment are indicated by A and the simulationresults of the comparative example are indicated by B. FIG. 8 showssimulation results of a three-bit variable capacitance circuit. Further,a maximum combined capacitance value is fixed.

It is seen from FIG. 8 that the Quality factor performance in theconfiguration of the embodiment is better than that in the comparativeexample. When the capacity is small, in particular, the Quality factorperformance in the former is significantly better than that in thelatter. This is because the switch resistance of the NMOS transistor M1when it is in the off state is finite and the Quality factor stronglydepends on the resistance component. Now, the Quality factor when allthe unit capacity cells 51 of the three-bit variable capacitance circuit50 are OFF will be obtained.

The switch resistance when the NMOS transistor M1 is in the off state isdenoted by R_(OFF) . When the Quality factor in the configuration of thecomparative example is denoted by Q_(CONV) the following Expression (7)is obtained.

$\begin{matrix}{Q_{conv} = \frac{1}{\omega \cdot C_{u} \cdot R_{OFF}}} & (7)\end{matrix}$

In a similar way, when the Quality factor of the configuration of thefirst embodiment is denoted by Q_(propl), the following Expression (8)is obtained.

$\begin{matrix}\begin{matrix}{Q_{{prop}\; 1} = \frac{1 + {2 \cdot \left( {\omega \cdot M \cdot C_{u} \cdot R_{OFF}} \right)^{2}}}{\omega \cdot M \cdot C_{u} \cdot R_{OFF}}} \\{= {2 \cdot \left( {\omega \cdot M \cdot C_{u} \cdot R_{OFF}} \right)}}\end{matrix} & (8)\end{matrix}$

Since R_(OFF) is an OFF switch resistance, it has a high resistance.

From Expressions (7) and (8), it can be seen that the Quality factor inthe first embodiment is larger than that in the comparative example.

As described above, in this embodiment, the AC voltage applied to thedrain terminal of the NMOS transistor M1 is 1/ (1+M) times larger thanthe output AC voltage. It is therefore possible to reduce the voltageamplitude in the drain terminal. It is therefore possible to achieve ahigh output transmission and to prevent the malfunction of the variablecapacitance circuit. Further, it is possible to reduce the leak currentwhen the NMOS transistor M1 is in the off state. Even when the variablecapacitance circuit 50 having a small capacitance is used, the Qualityfactor performance can be improved. As described above, according tothis embodiment, it is possible to provide the variable capacitancecircuit 50 having a high performance.

It is possible to use the variable capacitance circuit 50 according tothis embodiment in the capacitor C1 and the capacitor C2 shown in FIG.2. According to the above configuration, there is no need to use thehigh breakdown voltage element as the NMOS transistor M1 also in theRFIC 100 that outputs a large voltage amplitude. It is thereforepossible to form the NMOS transistor M1 by the thin-film MOS transistorhaving a low breakdown voltage. Since the thin-film MOS transistor thesame as the main circuit can be used, there is no need to provide anadditional power supply and an additional voltage source circuit.

Second Embodiment

With reference to FIGS. 9 and 10, a variable capacitance circuit 50according to this embodiment will be described. FIG. 9 is a circuitdiagram showing a configuration of the variable capacitance circuit 50and FIG. 10 is a circuit diagram showing a configuration of a unitcapacity cell 51 of the variable capacitance circuit 50.

In the second embodiment, a resistor R1, a transmission gate TG1, a NOTcircuit NT1, and a bias voltage input terminal VB are added to the unitcapacity cell 51 shown in FIG. 7. Since the basic configurations of thevariable capacitance circuit 50 and the unit capacity cell 51 aresimilar to those in the first embodiment, descriptions thereof will beomitted.

The bias voltage input terminal VB is connected to the firstintermediate terminal N1 via the transmission gate TG1 and the resistorR1. The first intermediate terminal N1 is a terminal between the firstcapacitor Cu1 and the second capacitor Cu2. The bias voltage inputterminal VB is connected to the transmission gate TG1. Therefore, thebias voltage is input to the transmission gate TG1. One end of theresistor R1 is connected to the output of the transmission gate TG1 andthe other end of the resistor R1 is connected to the first intermediateterminal N1. Therefore, the output side of the transmission gate TG1 isconnected to the drain terminal of the NMOS transistor M1 via theresistor R1.

The negative control terminal of the transmission gate TG1 is connectedto the control input terminal b0. The positive control terminal of thetransmission gate TG1 is connected to the output of the NOT circuit NT1.The control terminal b0 is connected to the input side of the NOTcircuit NT1. Therefore, the transmission gate TG1 serves as a switchthat is turned on or off by a control signal. When the transmission gateTG1 is turned on, the bias voltage is supplied to the first intermediateterminal N1 and to the drain terminal of the NMOS transistor M1 via theresistor R1.

Next, the active operation of the unit capacity cell 51 will bedescribed. When the unit capacity cell 51 shown in FIG. 10 is in the on(active) state, the NMOS transistor M1 is in the on state and the switchresistance becomes zero. In this case, the control signal of the controlinput terminal b0 is H and the output of the NOT circuit NT1 is L.Therefore, the transmission gate TG1 is in the high-impedance state. Thebias voltage is not supplied to the drain terminal of the NMOStransistor M1. Therefore, the capacitance value of the unit capacitycell 51 is Cu.

On the other hand, when the unit capacity cell is in the off (inactive)state, the NMOS transistor M1 is in the off state and the switchresistance becomes infinite. In this case, the control signal of thecontrol input terminal b0 is L and the output of the NOT circuit NT1 isH. Therefore, the transmission gate TG1 is in the conduction state.Therefore, the bias voltage is supplied to the drain terminal of theNMOS transistor M1. Therefore, the capacitance value of the unitcapacity cell 51 becomes a combined capacitance of the two seriescapacitances. That is, the capacitance value of the unit capacity cell51 becomes M/(1+M)·Cu.

In the configuration according to the second embodiment, when the unitcapacity cell 51 is in the off state, the level of the drain potentialof the NMOS transistor M1 is shifted by the amount corresponding to thebias voltage. It is therefore possible to accept the amplitude largerthan that in the first embodiment. In the following description, thedrain voltage when the unit capacity cell 51 is turned off will beconsidered.

The DC bias voltage becomes VB and the signal voltage amplitude becomesthe amplitude in which the voltage is attenuated compared to that shownby Expression (6). When the voltage amplitude applied to the variablecapacitance circuit 50 is denoted by Vrf, the following Expression (9)is obtained.

$\begin{matrix}{{{VB} + {\frac{1}{1 + M} \cdot {Vrf}}} \geq {V(D)} \geq {{VB} - {\frac{1}{1 + M} \cdot {Vrf}}}} & (9)\end{matrix}$

Similar to the comparative example, the bias voltage and the allowablemaximum voltage amplitude to make the voltage amplitude maximum areobtained. The maximum value of the drain voltage of the NMOS transistorM1 is determined by the element breakdown voltage Vmax of the OFFtransistor. On the other hand, the minimum value of the drain voltage ofthe NMOS transistor M1 is determined under the condition that themalfunction in which the MOS switch becomes ON does not occur andbecomes −V_(TH). Therefore, with the use of Expression (9), the maximumamplitude and the bias voltage are respectively expressed by thefollowing Expressions (10) and (11).

$\begin{matrix}{{Vrf} = {\frac{1 + M}{2}\left( {{V\; \max} + V_{TH}} \right)}} & (10) \\{{VB} = {\frac{1}{2}\left( {{V\; \max} - V_{TH}} \right)}} & (11)\end{matrix}$

The maximum amplitude according to the first embodiment becomes equal tothe threshold voltage of the NMOS transistor M1. Therefore, in thesecond embodiment, from the expression (10), it is possible to acceptthe amplitude (1+M)/2·(1+Vmax/V_(TH)) times larger than that in thefirst embodiment. Accordingly, it is possible to make the allowablevoltage amplitude larger, whereby a higher output transmission can beperformed.

Third Embodiment

With reference to FIGS. 11 and 12, a variable capacitance circuit 50according to this embodiment will be described. FIG. 11 is a circuitdiagram showing a configuration of the variable capacitance circuit 50and FIG. 12 is a circuit diagram showing a configuration of a unitcapacity cell 51 of the variable capacitance circuit 50.

The variable capacitance circuit 50 according to this embodiment isobtained by employing a differential capacitance in the variablecapacitance circuit 50 according to the second embodiment. Therefore,the output terminal OUTN is not connected to the ground. That is, theoutput terminal OUTN is a negative differential output and the outputterminal OUTP is a positive differential output.

Further, a first capacitor Cu1, a second capacitor Cu2, and a thirdcapacitor Cu3 are connected in series between the output terminal OUTPand the output terminal OUTN. Further, a resistor R2 is added. Since theconfiguration other than the third capacitor Cu3 and the resistor R2 aresimilar to that of FIG. 10, a description thereof will be omitted.

The first capacitor Cu1 corresponds to the first capacitor Cu1 shown inFIG. 10 and the second capacitor Cu2 corresponds to the second capacitorCu2 shown in FIG. 10. Accordingly, in the third embodiment, the thirdcapacitor Cu3 is added to the output terminal OUTN side of the secondcapacitor Cu2.

Specifically, one end of the first capacitor Cu1 is connected to theoutput terminal OUTP and the other end of the first capacitor Cu1 isconnected to one end of the second capacitor Cu2. One end of the thirdcapacitor Cu3 is connected to the other end of the second capacitor Cu2and the other end of the third capacitor Cu3 is connected to the outputterminal OUTN. The terminal between the first capacitor Cu1 and thesecond capacitor Cu2 is referred to as a first intermediate terminal N1and the terminal between the second capacitor Cu2 and the thirdcapacitor Cu3 is referred to as a second intermediate terminal N2.

The NMOS transistor M1 is connected in parallel with the secondcapacitor Cu2. Therefore, the drain terminal of the NMOS transistor M1is connected to the first intermediate terminal N1 and the sourceterminal thereof is connected to the second intermediate terminal N2.

The output of the transmission gate TG1 is connected to the secondintermediate terminal N2 via the resistor R2. Therefore, when thetransmission gate TG1 is turned on, the bias voltage is supplied to thesecond intermediate terminal N2 via the resistor R2.

The control terminal b0 is connected to the gate terminal of the NMOStransistor M1 and the input of the NOT circuit NT1. The bias voltageinput terminal VB is connected to the drain terminal of the NMOStransistor M1 via the transmission gate TG1 and the resistor R1.Further, the bias voltage input terminal VB is connected to the sourceterminal of the NMOS transistor M1 via the transmission gate TG1 and theresistor R2. Further, the positive control terminal of the transmissiongate TG1 is connected to the output of the NOT circuit NT1 and thenegative control terminal thereof is connected to the control terminalb0.

The capacitance value of the first capacitor Cu1 and the third capacitorCu3 is denoted by Cu and the capacitance value of the second capacitorCut is denoted by 1/2·M·Cu.

Next, a description will be given of an active operation according tothe third embodiment. When the unit capacity cell 51 shown in FIG. 12 isin the on (active) state, the NMOS transistor M1 is in the on state andthe switch resistance becomes zero. In this case, the control signal ofthe control terminal b0 is H and the output of the NOT circuit NT1 is L.Therefore, the transmission gate TG1 is in the high-impedance state.Therefore, the bias voltage input to the bias voltage input terminal VBis not supplied to the drain terminal of the NMOS transistor M1. Thedifferential combined capacitance value of the unit capacity cell 51becomes 1/2·Cu. That is, the first capacitor Cu1 and the third capacitorCu3 are connected in series via the NMOS transistor M1 which is in theON state. Therefore, the serial combined capacitance value of the firstcapacitor Cu1 and the third capacitor Cu3 becomes the capacitance valueof the unit capacity cell 51.

On the other hand, when the unit capacity cell is in the off (inactive)state, the NMOS transistor M1 is in the off state and the switchresistance becomes infinite. In this case, the control signal of thecontrol terminal b0 is L and the output of the NOT circuit NT1 is H.Therefore, the transmission gate TG1 is in the conduction state.Therefore, the bias voltage is supplied to each of the drain terminaland the source terminal of the NMOS transistor M1 via the resistors R1and R2. Therefore, the capacitance value of the unit capacity cell 51becomes the serial combined capacitance value of the first capacitorCu1, the second capacitor Cu2, and the third capacitor Cu3 and becomes(M/(1+2M)·Cu).

In this embodiment, the signal voltage between the drain and the voltageapplied to the NMOS transistor M1 can be attenuated. When the unitcapacity cell 51 shown in FIG. 12 is in the inactive state, the unitcapacity cell 51 can be regarded as a circuit in which the firstcapacitor Cu1, the second capacitor Cu2, and the third capacitor Cu3 areconnected in series. When the voltage applied to the variablecapacitance circuit 50 is denoted by V (OUT), the voltage between thedrain and the source has a value the same as that in Expression (6).

On the other hand, when a general differential variable capacitancecircuit is used, the voltage between the drain and the source in theinactive state becomes V(OUT). Therefore, according to the configurationof the third embodiment, it is possible to attenuate the voltage signalbetween the drain and the source. It is therefore possible to preventthe element breakdown voltage of the transistor and the malfunction ofthe MOS switch.

While the variable capacitance circuit 50 according to this embodimentis obtained by employing the differential capacitance in theconfiguration of the second embodiment, it may be obtained by employingthe differential capacitance in the configuration of the firstembodiment. In the latter case, the third capacitor Cu3 is added to theconfiguration of FIG. 7. That is, the third capacitor Cu3 is arrangedbetween the second capacitor Cu and the output terminal OUTN.

Fourth Embodiment

With reference to FIG. 13, a variable capacitance circuit according tothis embodiment will be described. FIG. 13 is a circuit diagram showinga configuration of a unit capacity cell 51 of a variable capacitancecircuit 50. In this embodiment, the second capacitor Cut of the unitcapacity cell 51 according to the second embodiment is replaced by animpedance element Zb. Since the other configuration is similar to thatin the second embodiment, a description thereof will be omitted.

The first capacitor Cu1 and the impedance element Zb are connected inseries between the output terminal OUTP and the output terminal OUTN.Therefore, when the unit capacity cell 51 is in the off (inactive)state, the NMOS transistor M1 is turned off. Therefore, it can be saidthat the unit capacity cell 51 is a circuit in which the first capacitorCu1 and the impedance element Zb are connected in series. Therefore, thedrain terminal voltage of the NMOS transistor M1 is divided by the firstcapacitor Cu and the fixed impedance Zb and the voltage is attenuated.It is therefore possible to obtain the effect similar to that in thesecond embodiment.

The impedance element Zb may be, for example, a resistor. In alow-frequency analog circuit or the like, a resistor may be used as theimpedance element Zb. While the second capacitor Cu2 according to thesecond embodiment has been replaced by the impedance element Zb in FIG.13, the second capacitor Cu2 according to the first embodiment or thethird embodiment may be replaced by the impedance element Zb.

The first to fourth embodiments stated above may be combined asappropriate.

(Example to Which Variable Capacitance Circuit is Applied)

While the example in which the variable capacitance circuit 50 isapplied to the capacitor C1 of the HPA circuit 43 has been described inthe above embodiments, the variable capacitance circuit 50 may also beapplied to other capacitors. It is possible to apply the variablecapacitance circuit 50, for example, to the capacitor C2 of the bandreject filter 44 shown in FIG. 2.

Besides an amplification circuit, the variable capacitance circuit 50can be used for a matching circuit, a load circuit, a front-end circuit,and a resonant circuit. The load circuit includes circuits, for example,provided after the amplifier 47 in FIG. 2. Therefore, one or a pluralityof the capacitor C1 and the capacitor C2 may be the variable capacitancecircuit 50. The front-end circuit includes the first-stage LNA circuit24 and the circuits provided before the first-stage LNA circuit 24 inthe receiver 20. For example, the internal matching circuit 23 shown inFIG. 1 is the front-end circuit. The capacitor of the internal matchingcircuit 23 may be the variable capacitance circuit 50. It is thereforepossible to solve the problem of the element breakdown voltage and toprevent the malfunction of the variable capacitance circuit.

Furthermore, the variable capacitance circuit 50 can be applied also tothe balun resonant circuit. When the inductors L1 and L2 and thecapacitor C1 shown in FIG. 2 are balun resonant circuits, for example,the variable capacitance circuit 50 can be used for the capacitor C1.Furthermore, the variable capacitance circuit 50 may be connected to theinductor L1 on the input side in parallel. Alternatively, the variablecapacitance circuit 50 may be connected to both the inductor L1 and theinductor L2 in parallel. It is therefore possible to solve the problemof the element breakdown voltage and to prevent the malfunction of thevariable capacitance circuit.

Further, the variable capacitance circuit 50 can be applied also to theserial resonant circuit in which the inductor and the capacitor areconnected in series. When the inductor L3 and the capacitor C2 shown inFIG. 2 are serial resonant circuits, for example, the variablecapacitance circuit 50 can be used for the capacitor C2. It is thereforepossible to solve the problem of the element breakdown voltage and toprevent the malfunction of the variable capacitance circuit.

Further, the variable capacitance circuit 50 can be applied also to theparallel resonant circuit in which the capacitor and the inductor areconnected in parallel. When the inductor L2 and the capacitor C1 shownin FIG. 2 are parallel resonant circuits, for example, the variablecapacitance circuit 50 may be used as the capacitor C1. In theoscillation circuit, the parallel resonant circuit can be used as aload. In the oscillation circuit that uses the parallel resonant circuitas the load circuit, a large voltage amplitude can be output.

Further, the variable capacitance circuit 50 can be applied also to theLNA circuit 24 and the VCO circuit 42 including the system configurationthat outputs a large voltage amplitude. Similar to the HPA circuit 43,it is possible to solve the problem of the element breakdown voltage andto prevent the malfunction of the variable capacitance circuit also inthe LNA circuit 24 and the VCO circuit 42.

FIG. 14 shows a configuration example of the LNA circuit 24. The LNAcircuit 24 includes an inductor L11, a capacitor C11, an NMOS transistorM11, and an NMOS transistor M12. The inductor L11 and the capacitor C11are connected in parallel. Further, the drain terminal of the NMOStransistor M11 is connected to one end of the capacitor C11. Further,the drain terminal of the NMOS transistor M12 is connected to the otherend of the capacitor C11. The source terminals of the NMOS transistorsM11 and M12 are connected to the ground.

The output terminal OUTP is connected to one end of the capacitanceelement C11 and the output terminal OUTN is connected to the other end.The variable capacitance circuit 50 stated above can be used for such acapacitor C11 of the LNA circuit 24. When the LNA circuit 24 is adifferential output, for example, the variable capacitance circuit 50according to the third embodiment can be used for the capacitor C11.

FIG. 15 shows a configuration example of the VCO circuit 42. The VCOcircuit 42 includes an inductor L21, a capacitor C21, an NMOS transistorM21, and an NMOS transistor M22. The inductor L21 and the capacitor C21are connected in parallel. Further, the drain terminal of the NMOStransistor M21 is connected to one end of the capacitor C21. Further,the drain terminal of the NMOS transistor M22 is connected to the otherend of the capacitor C21. The source terminals of the NMOS transistorsM21 and M22 are connected to the ground. Further, the gate terminal ofthe NMOS transistor M21 is connected to the intermediate terminal N22.The intermediate terminal N22 is a terminal which is provided betweenthe drain terminal of the NMOS transistor M22 and the capacitor C21. Thegate terminal of the NMOS transistor M22 is connected to theintermediate terminal N21. The intermediate terminal N21 is a terminalwhich is provided between the drain terminal of the NMOS transistor M22and the capacitor C21.

The output terminal OUTP is connected to one end of the capacitanceelement C21 and the output terminal OUTN is connected to the other endthereof. The variable capacitance circuit 50 stated above can be usedfor the capacitor C21 of the VCO circuit 42. When the VCO circuit 42 isa differential output circuit, for example, the variable capacitancecircuit 50 according to the third embodiment may be used as thecapacitor C21.

As described above, the variable capacitance circuit 50 can be used forthe capacitor used in various circuits such as a serial resonantcircuit, a parallel resonant circuit, a balun resonant circuit, a loadcircuit, an oscillation circuit, a front-end circuit and the like. Inthis embodiment, it is possible to attenuate the signal voltage appliedto the NMOS transistor M1 of the unit capacity cell 51. It is thereforepossible to suppress the problems of the breakdown voltage of the NMOStransistor M1 and the leak current. It is possible to prevent themalfunction of the digital variable capacitor. A low breakdown voltageelement having a low breakdown voltage can be used as the NMOStransistor M1. It is possible to output a large output amplitude.Further, it is possible to improve the capacity performance (Qualityfactor) compared to the case in which the high breakdown voltage MOStransistor is used for the NMOS transistor M1. Even in the case of thehigh output transmission, the high breakdown voltage MOS transistor maynot be used, whereby it is possible to reduce the power supply voltage.

As described above, it is possible to provide the variable capacitancecircuit 50 having a high performance. Further, the performance ofvarious circuits such as the serial resonant circuit, the parallelresonant circuit, the balun resonant circuit, the load circuit, theoscillation circuit, and the front-end circuit can be improved. Thevariable capacitance circuit 50 is suitable for a transmitter in theradio communication. The variable capacitance circuit 50 is suitable fora battery-driven transmitter or the like since the power supply voltageof the variable capacitance circuit 50 can be decreased. Specifically,the variable capacitance circuit 50 is suitable for the RFIC used in thesub-GHz radio communication or the RFIC that complies with the BLE. Thatis, since the transmitter including the variable capacitance circuit 50can be driven by the alkaline battery, economy can be improved.

(Configuration of MOS Transistor)

As described above, an element whose breakdown voltage is low can beused as the NMOS transistor M1. It is thus possible to use the thin-filmtransistor having a low breakdown voltage as the NMOS transistor M1.FIG. 16 shows a cross-sectional configuration of a thick-film transistorand a thin-film transistor.

As shown in FIG. 16, a thin-film transistor 98 and a thick-filmtransistor 99 are formed on a substrate 90. The thin-film transistor 98is arranged in a thin-film transistor forming region 901. Further, thethick-film transistor 99, which is a high breakdown voltage MOStransistor, is arranged in a thick-film transistor forming region 902.The thin-film transistor 98 and the thick-film transistor 99 are MOStransistors and the thickness of the gate insulating film 94 variesbetween the thin-film transistor 98 and the thick-film transistor 99.

Each of the thin-film MOS transistor 98 and the thick-film transistor 99includes an N-type source 91, an N-type drain 92, a gate 93, and a gateinsulating film 94. The gate insulating film 94 is arranged on thesubstrate 90 between the source 91 and the drain 92. Further, the gate93 is arranged on the gate insulating film 94. The gate insulating filmof the thin-film MOS transistor 98 is referred to as a gate insulatingfilm 94 a and the gate insulating film of the thick-film MOS transistor99 is referred to as a gate insulating film 94 b.

The gate insulating film 94 a is thinner than the gate insulating film94 b. Therefore, the breakdown voltage of the thin-film MOS transistor98 is lower than that of the thick-film transistor 99. In other words,the thick-film transistor 99 is a high breakdown voltage element and thethin-film transistor 98 is a low breakdown voltage element. The NMOStransistor M1 of the variable capacitance circuit 50 is formed of thethin-film transistor 98.

Therefore, the circuit that uses the variable capacitance circuit 50(e.g., transistors such as the HPA circuit 43, the VCO circuit 42, theLNA circuit 24 and the like shown in FIG. 2) can be formed of thethin-film transistor 98. It is therefore possible to reduce the powersupply voltage and to drive the RFIC 100 by the alkaline battery (1.6V). The variable capacitance circuit 50 is therefore suitable for theSub-GHz RFIC that complies with IEEE 802.15.4 g and the RFIC thatcomplies with the BLE. By using the variable capacitance circuit 50 forthe communication apparatus 1, the communication apparatus 1 can becomplied with the standards of various countries. Only the circuit thatrequires a high breakdown voltage (e.g., transistors of the band rejectfilter 44 and the internal matching circuit 23) may be formed of thethick-film transistor 98.

While the invention made by the present inventors has been specificallydescribed based on the embodiments, it is needless to say that thepresent invention is not limited to the embodiments stated above and maybe changed in various ways without departing from the spirit of thepresent invention.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention can bepracticed with various modifications within the spirit and scope of theappended claims and the invention is not limited to the examplesdescribed above.

Further, the scope of the claims is not limited by the embodimentsdescribed above.

Furthermore, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

1. A digital variable capacitance element comprising a plurality ofcapacity cells that are connected in parallel between two outputterminals, wherein the capacity cell comprises: a first capacitor havingone end connected to one of the two output terminals; an impedanceelement that is connected in series with the first capacitor between thetwo output terminals; and a transistor that is connected in parallelwith the impedance element and is controlled in accordance with adigital control signal.
 2. The digital variable capacitance circuitaccording to claim 1, wherein the impedance element is a secondcapacitor.
 3. The digital variable capacitance circuit according toclaim 1, further comprising: a bias voltage supply terminal that issupplied with a bias voltage; a switch that supplies the bias voltage toan intermediate terminal between the first capacitor and the impedanceelement in accordance with the digital control signal; and a resistorarranged between the intermediate terminal and the bias voltage supplyterminal.
 4. The digital variable capacitance circuit according to claim1, wherein: the capacity cell further comprises a third capacitor, thefirst capacitor, the impedance element, and the third capacitor areconnected in series in this order between the two output terminals, anda differential operation is performed by the two output terminals. 5.The digital variable capacitance circuit according to claim 4, furthercomprising: a bias voltage supply terminal that is supplied with a biasvoltage; a switch that supplies the bias voltage to first and secondintermediate terminals in accordance with the digital control signal; afirst resistor that is arranged between the first intermediate terminaland the bias voltage supply terminal; and a second resistor that isconnected between the second intermediate terminal and the bias voltagesupply terminal, wherein: the first intermediate terminal is anintermediate terminal between the first capacitor and the impedanceelement, and the second intermediate terminal is an intermediateterminal between the third capacitor and the impedance element.
 6. Abalun resonant circuit comprising: the digital variable capacitancecircuit according to claim 1; a first inductor; and a second inductorthat is coupled with the first inductor, wherein the digital variablecapacitance circuit is connected in parallel with one of the firstinductor and the second inductor.
 7. A serial resonant circuitcomprising: the digital variable capacitance circuit according to claim1; and an inductor that is connected in series with the digital variablecapacitance circuit.
 8. A parallel resonant circuit comprising: thedigital variable capacitance circuit according to claim 1; and aninductor that is connected in parallel with the digital variablecapacitance circuit.
 9. An oscillation circuit that uses the parallelresonant circuit according to claim 8 as a load.
 10. An amplificationcircuit comprising: the balun resonant circuit according to claim 6; aserial resonant circuit comprising an inductor that is connected inseries with the digital variable capacitance circuit; and a parallelresonant circuit comprising an inductor that is connected in parallelwith the digital variable capacitance circuit.
 11. A transmittercomprising the digital variable capacitance circuit according to claim1.